Reflection and Refraction

1. Reflection and Refraction

2. Light interacts with matter • Interaction begins at surface and depends on • Smoothness of surface • Nature of the material • Angle of incidence • Possible interactions • Reflection • Refraction • Absorption • Transmission • Transparent materials transmit light • Opaque materials do not allow transmission of light • Reflect, absorb or combination

3. Reflection • Most objects we see reflect light rather than emit their own light.

4. Principle of Least Time • Fermat's principle - light travels in straight lines and will take the path of least time A B Wrong Path True Path MIRROR

5. Law of Reflection • “The angle of incidence equals the angle of reflection.” • This is true for both flat mirrors and curved mirrors.

6.  =  i r Reflection • Angles measured with respect to the “surface normal” • Line perpendicular to the surface Law of Reflection Angle of incidence = Angle of reflection

7. Image formation • Real image • Can be viewed or displayed at its location • Example - movie image on a screen • Virtual image • Appears to come from a location where it is not directly visible • Examples: plane mirror, convex mirror, concave mirror

10. Types of Reflection • Specular Reflection - images seen on smooth surfaces (e.g. plane mirrors) • Diffuse Reflection - diffuse light coming from a rough surface (cannot see a reflection of yourself)

11. Refraction • Refraction is the bending of light when it passes from one transparent medium to another. • This bending is caused by differences in the speed of light in the media.

12. Refraction Light Beam Normal Line Fast AIR WATER Slow

13. Refraction • Light crossing a boundary surface and changing direction • Reason: change in light propagation speed • Moving to a medium with a slower propagation speed • Light bends toward surface normal • Moving to a medium with a faster propagation speed • Light bends away from the normal • Magnitude of refraction depends on: • Angle that light strikes surface • Ratio of speed of light in the two transparent materials • Incident ray perpendicular to surface not refracted

14. Refraction Examples • Light slows down when it goes from air into water and bends toward the normal. • An Analogy: A car slows down when it goes from pavement onto gravel and turns toward the normal. • An Illusion : Fish in the water appear closer and nearer the surface. (See Figure 28.27)

15. Refraction Observer AIR WATER False Fish True Fish

16. Atmospheric Refraction • Our atmosphere can bend light and create distorted images called mirages.

17. Refraction • Mirages • Critical angle • Light refracted parallel to surface • No light passes through surface - “total internal reflection” • Applications - fiber optics, gemstone brilliance • Index of refraction • A measure of light speed

18. Total Internal Reflection... • …is the total reflection of light traveling in a medium when it strikes a surface of a less dense medium

19. AIR Critical Angle WATER Total Internal Reflection Light Source

21. Lenses • Converging Lens • A lens that is thicker in the middle and refracts parallel light rays passing through to a focus. • Diverging Lens • A lens that is thinner in the middle than at the edges, causing parallel light rays to diverge.

22. Optics • The use of lenses to form images • Concave lenses • Diverging lenses • Vision correction/in association with other lenses • Convex lenses • Converging lenses • Most commonly used lens • Magnifiers, cameras, eyeglasses, telescopes, …

24. Myopia (Near-Sightedness) People with near-sightedness cannot see clearly at distance.

25. Hyperopia (Farsightedness) People with far-sightedness cannot see clearly up close.

26. The human eye • Uses convex lens with muscularly controlled curvature to change focal distance • Nearsightedness (myopia) - images form in front of retina • Farsightedness (hyperopia) - images form behind retina • Correction - lenses (glasses, contacts) used to move images onto retina

27. Total Internal Reflection occurs when the angle of incidence is greater than the critical angle. • The critical angle for an air-water interface is 48°. • Fish can see everything above the water’s surface in a 96° cone of vision.

28. Dispersion... • …is the separation of white light into pure colors (ROY G. BIV). • Dispersion Examples: • Prisms • Diffraction Gratings • CD’s • Raindrops

30. Dispersion and colors • White light • Mixture of colors in sunlight • Separated with a prism • Dispersion • Index of refraction varies with wavelength • Different wavelengths refract at different angles • Violet refracted most (blue sky) • Red refracted least (red sunsets) • Example: rainbows • Wavelength/frequency related

31. Rainbows • Raindrops refract, reflect and disperse sunlight. • Rainbows will always appear opposite of the Sun in the sky. • You cannot run from or run to a rainbow!

32. Rainbows

33. The nature of lightWave-like behavior Interference • Young’s two slit experiment • Interference pattern - series of bright and dark zones • Explanation - constructive and destructive interference

34. Wave-like behavior - polarization • Alignment of electromagnetic fields • Unpolarized light - mixture of randomly oriented fields • Polarized light - electric fields oscillating on one direction • Two filters - passage depends on alignment • Reflection polarization

35. Polarization • The alignment of the electric vectors that make up electromagnetic radiation. • See Figure 29.31.

36. Polarization • Light can be polarized by: • (1) passing light though a polarizer material or • (2) reflecting light off of a solid or liquid surface. • For example, reflected light from a lake is mostly horizontally polarized.